This invention relates to the production of a transparent article and, more particularly, to the production of a shaped, transparent thermoplastic article, such as a container or bottle, having an incompatible filler, preferably a gas barrier strengthening filler dispersed therein, wherein the light absorption of the article has been altered to effectively mask or reduce the visual haze of the article.
Thermoplastic polymers, such as polyesters, have long been used in the production of packaging materials, including preforms which are then blown or otherwise oriented into a desired form as necessary for the production of plastic articles such as containers and/or bottles for food and beverage storage and delivery. Among the most preferred and cost-effective thermoplastic polymers used for this purpose are poly(ethylene phthalate) resins. Poly(ethylene terephthalate) (PET), as well as other polyesters, when processed properly under the right conditions and oriented into a desired shape, provides a high clarity, low haze article. Consequently, the plastic bottling industry has used PET and similar polyesters for several years in its production of plastic containers and bottles for food and beverages.
Unfortunately, while plastic containers made from polyester, provide excellent high strength containers having excellent gas barrier properties for most foods and beverages, they are presently not suitable as beer containers or other food containers where extremely low gas permeability is required. It will be appreciated that when oxygen and other air gases come into contact with certain foods and beverages, such as beer for example, the beer oxidizes or otherwise becomes stale. Consequently, attempts have been made heretofore to reduce the oxygen/gas permeability of the container or, stated another way, to increase the gas barrier strength of the container.
One known way to reduce oxygen/gas permeability or to increase the gas barrier strength of the container is to blend certain gas barrier strengthening fillers with the polyester in the container. For instance, certain polyamides, such as polyxylylene amides, are well known in the art to provide improved gas barrier strength to polyester containers. To produce these containers, the filler is typically blended or dispersed in the polyester by processes known in the art and then the article is manufactured. In some instances, the containers may be molded as by injection molding and the like. In other instances, preforms of the containers are prepared such as by injection molding or extrusion, and are then blown or otherwise oriented into the desired size and shape.
Various patents and patent publications have taught the use of polyester/polyamide blend compositions for forming an article having low haze and reduced gas permeability compared to polyester alone. In at least one patent publication, in order to provide a low haze/low gas permeability container, it is stated that the blend composition employ a polyamide having a number average molecular weight of less than 15,000. That patent publication further makes it clear that blends of higher molecular weight polyamides with polyester are known to have high haze values which limit their practical use in the food and beverage container industry.
In other words, heretofore, few, if any, blends of polyester and these gas barrier strengthening fillers, such as higher molecular weight polyamides, have been used in the plastic container or bottling industry, or any industry where transparent, high clarity articles are desired, because it is a well-known fact that, upon orienting or stretching an article containing a blend of polyester and polyamide, the article loses much of its clarity and transparency, i.e., becomes visually cloudy or hazy. This characteristic is known in the industry as haze.
Haze, as described in most of the patent literature, can be measured, much like any other physical property. Measurements to determine the level or amount of haze may be obtained using a calorimeter (e.g., Hunter Lab Color Quest) and following ASTM D1003. Haze is typically reported as a percentage based upon the thickness of the article and can be calculated by the equation
      Haze    ⁢                  ⁢    %    =                    T        Diffuse                    T        Total              ×    100  where Haze % equals transmittance haze, TDiffuse equals diffuse light transmittance, and TTotal equals total light transmittance. A 4% haze measurement in a container sidewall approximately 15 mils thick is normally visible to the naked eye. Generally, when testing containers made from different blends of polyester and polyamides, haze values have been measured in the 15% to 35% range for these 15 mil thick containers. For purposes of this invention, this type of haze will often be referred to hereinafter as “physical haze” or “measured haze.”
Moreover, as the amount of gas barrier strengthening filler used in the polyester/filler blend increases, the physical haze value also increases. In fact, it has been found by others that effective blend ratios of polyester (e.g., PET) and aromatic polyamides (e.g., poly(m-xylylene adipamide) commonly referred to as MXD6) provide for physical haze values in the 20% to 30% range upon orienting the polymers into the form of a container again having a wall thickness of about 15 mils.
Heretofore, efforts have focused on reducing the gas permeability of the article by addition of gas barrier strengthening fillers, while, at the same time, trying to reduce the amount of physical haze produced upon orientation of the article. Such efforts, where successful, have generally found that to reduce physical haze, the size of the molecules of the filler had to be significantly small. Generally, it is understood, as stated above, that polyamides having a number average molecular weight of less than 15000 in a concentration of less than 2 percent by weight are needed to sufficiently reduce physical haze. Alternatively, it has been found that, where polyamide domains in the polyester have been limited to an average number size of from 30 to 200 nanometers, physical haze will also be reduced or limited. At least one theory for this phenomenon is that the polyamide particles are so small that they fail to scatter light, particularly in the visible spectra, i.e., the particles do not reflect light to the observer in a manner detectable to the naked eye. Moreover, in measuring the physical haze using machines such as a calorimeter, it is clear that the physical haze measured has been reduced or potentially even eliminated.
Based upon this theory, it should be understood then that, where those particles or domains surrounding the filler are much larger than 200 nanometers, say on the order of 400 to 700 nanometers, the haze of the article is not only physically measurable, but also may be visible to the ordinary observer. In fact, at least one journal article expressly recognizes that the number and size of the dispersed particles does create measured haze. It is further noted therein that stretching makes for even more measured haze because, firstly, stretching increases the size of the dispersed particles in a sheet plane and, secondly, the difference in the anisotropic refractive indices of the matrix and the dispersed phase increases. Thus, some patents have attempted to prevent the stretching or reorienting of the MXD6 domains, for example, by producing bottles of PET and MXD6 when the polymer is in its molten state.
Hence, all of the prior art has focused on the physical haze phenomenon and the reduction or elimination thereof. In contrast, the present invention focuses on the visual aspect of the haze property since it is this characteristic which is believed to be detrimental to the cosmetic appearance and practical use of the article, not the physical haze of the article.
Heretofore, however, this “visual haze” or “visible haze” of an article has never been considered separate and apart from the physical haze of the article, as it is generally immeasurable by traditional physical testing of the article. By “visual haze” or “visible haze,” it is meant that haze which can be observed optically or visually by a person in ordinary direct or indirect light. It is the haze that is visible to the naked eye of the observer, presumably due to the reflectance or transmittance of the light from the filler domains present in the article. It is believed that the visual masking of the physical haze phenomenon results in the elimination or reduction of this “visual haze,” and can provide an article suitable for commercial use. To that end, it will be understood that “visual haze” is not a measured physical property to the same extent that the physical haze of an article is determinable on a calorimeter or the like, and eliminating or reducing visual haze may or may not reduce the measured physical haze of the article.
Accordingly, eliminating or reducing the “visual haze” of an article, regardless of the physical haze measurements, is seen as highly desirable to the art, particularly to the plastic container and bottling industry. Thus, there remains a need to provide a process by which to mask the visible haze of a transparent article made from polyester blended with a gas barrier strengthening filler, as well as for transparent, preferably oriented, articles comprising a polyester/filler blend that is aesthetically and visually acceptable to the plastic container and bottling industry.